The objective of this study is to obtain a detailed picture of cellular kinetics and lethality under conditions of continuous irradiation and to investigate whether and how the effects of continuous irradiation can be predictably reproduced by fractionated treatment in four different in vitro cell lines. The dose-rate effect due primarily to the accumulation of radiation damage and repair was studied by continuous irradiation at various dose rates at temperatures which slow down or prevent the movement of cells through the cycle. These studies will also include stationary cell populations in plateau phase. Dose-rate effects due to differences in the characteristics of the mitotic cycle will be studied by incubating cells during irradiation at temperatures between 28 degrees C and 37 degrees C, a procedure which essentially does not affect the ability of cells to accumulate and repair radiation damage, but which multiplies the duration of the mitotic cycle several times. The potential advantages of combining mild (39 degrees C - 42.5 degrees C) hyperthermia with continuous irradiation, as well as the effects of short (45 degrees C - 46 degrees C) heat treatments followed by continuous irradiation at various temperatures will be studied.